1. Field of the Invention
The present invention relates to a turbine starting controller capable of starting a turbine in the shortest time while keeping the thermal stress in a turbine rotor to a prescribed value or less and also relates to a turbine starting control method.
2. Description of the Related Art
In general, upon starting a steam turbine, the surface metal temperature of a turbine rotor rises as the heat transfer rate between the steam and the rotor improves due to a rise in the temperature of inflow steam and an increase in steam flow. The temperature of the inner portion of the turbine rotor rises due to the conduction of heat from the rotor surface at a timing delayed from the rise in the surface temperature. A deviation thus occurs in the temperature distribution in the inner portion of the turbine rotor, which will cause thermal stress. Since excessive thermal stress significantly reduces the life of the turbine rotor, the value of the generated thermal stress must be suppressed to an appropriate value or lower.
On the other hand, rapid and frequent start/stop operations are required of steam turbines in power generation plants in recent years. Unnecessarily rapid starting of the turbine may cause excessive thermal stress to be exerted on the turbine rotor. Accordingly, at the start of the steam turbine, a turbine starting control is required so as to make the consumption of rotor's life for one turbine startup appropriate while keeping the thermal stress on the turbine rotor within a limit value (prescribed value).
Further, it is known that the life consumption for one startup of the steam turbine can be grasped from the magnitude and the number of times of the thermal stress peaks at that time.
Japanese Patent Laid-open (KOKAI) Publication No. HEI 9-317404 describes a device for controlling the starting of a steam turbine so as to minimize the starting time of the steam turbine by limiting the thermal stress on the turbine rotor to a prescribed value or less.
In order to achieve optimized starting of the steam turbine, the turbine starting controller disclosed includes optimum pattern computing means, thermal stress predicting means, pattern correcting means, and manipulated variable adjusting means, forming a cascaded structure in which the calculated value computed by the optimum pattern computing means, the thermal stress predicting means, and the pattern correcting means serves as the set value for the manipulated variable adjusting means.
The conventional turbine starting controller is endowed with a thermal stress prediction starting control function for feedback-calculating the manipulated variable in the manipulated variable adjusting means so that, with the optimum rotor surface temperature transition pattern calculated on the basis of the predicted thermal stress serving as the set value, the measured value of plant state variables such as the temperature of the turbine rotor matches the set value. A drive control is performed on a flow rate adjusting valve by a turbine controller on the basis of the manipulated variable from the manipulated variable adjusting means, thereby controlling the starting of the steam turbine.
In the conventional turbine starting controller, the object of optimization calculation is the plant state variable such as the first stage metal temperature of the turbine rotor or the like, and the manipulated variable adjusting means is provided to calculate the turbine acceleration rate/load increase rate as the final manipulated variable. However, it is difficult to determine the manipulated variable (turbine acceleration rate/load increase rate) from a deviation in plant state variable, which may make it impossible to attain satisfactory optimization performance.
While the P control or the PI control is generally contemplated for the determination of the manipulated variable by the manipulated variable adjusting means, it is difficult to theoretically determine the control gain for computing the turbine acceleration rate/load increase rate as the final manipulated variable from a deviation in plant state variable.
Accordingly, with the starting control for the steam turbine according to which the drive of the control valve is controlled by the turbine controller on the basis of the manipulated variable computed by the manipulated variable adjusting means, it is difficult to match the plant state variable with the plant state variable set value obtained through the optimization calculation without time delay, which disadvantageously makes the original object of starting the turbine in the shortest time difficult to achieve in a satisfactory manner.
Further, in the conventional turbine starting controller, the optimum transition pattern for the plant state variable over a predetermined prediction period from the present to the future is calculated by the optimum pattern computing means, and the computed value (set value) at the current time in the computed optimum transition pattern is used for the determination of the manipulated variable by the manipulated variable adjusting means. Accordingly, the optimization calculation in the optimum pattern computing means is a calculation in which the plant state variable in each time step over the prediction period from the current time to the future is taken as a variable, so that if the prediction period consists of m steps, it means that the calculation is to be conducted for an m-variables optimization problem. Calculations involving multivariable optimization result in extremely high computational load (computational complexity), and the high computational complexity makes application to commercialization, that is, application to the actual turbine controllers difficult.